Homogeneous Charge Compression Ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer (typically air) are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction releases chemical energy into a form that can be translated by an engine into work and heat.
HCCI combustion has characteristics of two other and more popular forms of combustion used in internal combustion engines: homogeneous charge spark ignition (gasoline engines) and stratified charge compression ignition (diesel engines). As in homogeneous charge spark ignition, the fuel and air are mixed together. However, rather than using an electric discharge to ignite a portion of the mixture, compression is used to raise the pressure and temperature of the mixture until the entire mixture reacts spontaneously. In comparison, stratified charge compression ignition also relies on the temperature and pressure increase resulting from compression, but combustion occurs at the boundary of fuel-air mixing, caused by an injection event to initiate the combustion.
In an HHCI engine, because there is no direct initiator of combustion, HCCI combustion is inherently challenging to control. However, with advances in microprocessors and better physical understanding of the ignition process, HCCI is showing promise in achieving lowered emissions of gasoline engines emissions along with the better efficiency of diesel engines.